1. Field of the Invention
The present invention relates to a method for treating a combustion exhaust gas. More specifically, it relates to a method for recovering carbon dioxide (CO.sub.2) present in a combustion exhaust gas, while removing small amounts of ammonia present in the remaining exhaust gas after the recovery of CO.sub.2 by the use of an aqueous alkanolamine solution, and also a method for treating a combustion exhaust gas which has a denitration step for removing NO.sub.x (nitrogen oxides) present in the combustion exhaust gas and a CO.sub.2 removal step of removing CO.sub.2 using an aqueous alkanolamine solution in which ammonia (NH.sub.3) present in the combustion exhaust gas after the CO.sub.2 removal treatment is recovered and then used as a reducing agent in the denitration step, and further a method for removing CO.sub.2 from a combustion exhaust gas using an aqueous monoethanolamine (MEA) solution having a specific concentration as an absorbing agent.
2. Description of the Related Art
In recent years, a greenhouse effect by CO.sub.2 is indicated as one cause of the warming of the earth, and its prompt resolution is globally required in order to protect earth environment. Sources of CO.sub.2 extend over almost all fields of human activities in which fossil fuels are burned, and there is a tendency that regulations on the discharge of CO.sub.2 will be further tightened in the future. Thus, for power generation facilities such as thermoelectric power plants in which a large amount of the fossil fuel is used, intensive research efforts are being made on methods for recovering CO.sub.2 in a combustion exhaust gas by bringing the combustion exhaust gas coming from a boiler into contact with an aqueous alkanolamine solution or the like, and on methods for storing the recovered CO.sub.2 without discharging it into the atmosphere.
FIG. 7 shows one example of a process for recovering CO.sub.2 present in a combustion exhaust gas by the use of an aqueous alkanolamine solution. In FIG. 7, only major devices and parts are shown, and auxiliary devices are omitted.
Examples of alkanolamines which can be used to recover CO.sub.2 include aqueous solutions of monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine and diglycolamine, and aqueous mixed solutions thereof. In general, an aqueous solution of monoethanolamine is preferred.
In FIG. 7, a combustion exhaust gas which comes from a boiler or the like and which is to be discharged through a chimney has normally a temperature of 100.degree. to 150.degree. C., and this gas is introduced into a combustion exhaust gas cooling column 1 through line 5 and then brought into contact with a cooling water circulated through line 12 by a pump 11. The cooling water is cooled by a heat exchanger 13 and used repeatedly. Usually, the combustion exhaust gas cooled to a temperature in the range of about 40.degree. to 80.degree. C. is introduced into a CO.sub.2 absorbing column 2 through line 6.
An aqueous solution of an alkanolamine, e.g., monoethanolamine, at a concentration of 20 to 30% by weight is conducted to the CO.sub.2 absorbing column 2 through line 9. The aqueous monoethanolamine solution is brought into counter current contact with the combustion exhaust gas, and the aqueous monoethanolamine solution having absorbed CO.sub.2 is then fed from a lower portion of the column to a regeneration column 3 through line 7 for its regeneration. In order to prevent monoethanolamine from being discharged together with the remaining combustion exhaust gas (hereinafter referred to also as "the treated exhaust gas" at times) from which CO.sub.2 has been absorbed, a water washing and cooling section for the treated exhaust gas is provided in the upper portion of the CO.sub.2 absorbing column 2, and this section is equipped with a pump 15, a heat exchanger 16 and a tray 18 to which water is fed through line 19. The treated exhaust gas which has been washed and cooled with water in these devices is discharged to the atmosphere through line 8.
In the aqueous monoethanolamine solution regeneration column 3, the aqueous monoethanolamine solution is regenerated by heating the same with steam fed through a reboiler 4, and it is then returned to the absorbing column 2 through line 9. CO.sub.2 and water vapor are led together to a recovery step through line 10.
In addition, a cooling system for the combustion exhaust gas illustrated in FIG. 7 is mainly used for a fuel, such as LNG, containing a large amount of hydrogen. According to this cooling system, steam derived from the fuel and present in the combustion exhaust gas is cooled and becomes condensed water. This condensed water is accumulated as the cooling water circulating through line 12. Therefore, an excess of the cooling water is constantly discharged from the system through line 14 using a water level meter and the like (not shown) located in the lower portion of the combustion exhaust gas cooling column 1.
As a cooling system for the combustion exhaust gas, there is another system utilizing a humidification cooling technique suitable for boilers which use fuel containing a large amount of carbon, such as coal or fuel oil. In this system, no heat exchanger 13 is usually disposed, and circulating water is simply brought into contact with a combustion exhaust gas to cause the water evaporate for the purpose of cooling. Since the circulating water is gradually lost, it is necessary to replenish water from the outside.
In the above-mentioned process, ammonia is detected in the treated exhaust gas discharged through line 8, though its amount is very small. It is presumed that this ammonia in the treated exhaust gas is attributable to the decomposition of part of alkanolamine during the treatment processes. Another origin of the detected ammonia is considered to be residue of ammonia which has been added to the fuel for the purpose of decreasing the amount of NO.sub.x in the combustion exhaust gas. In any case, if ammonia, though its amount is very small, is discharged into the atmosphere together with the treated exhaust gas without any additional treatment, another environmental problem of bad odor may occur. Therefore, this ammonia has to be removed. However, ammonia is present in the treated exhaust gas in very small amounts, and thus how ammonia can effectively be removed has been a problem yet to be solved.
Further, when burned, fossil fuel generates pollutants, such as NO.sub.x (nitrogen oxides) and SO.sub.x (sulfur oxides), though the degree of this pollutant generation depends upon the kind of the fossil fuel. These pollutants cause air pollution and acid rain, and regulations on their discharge will be further tightened. With this current trend, measures have been taken so that a combustion exhaust gas coming from a boiler is treated by denitration and desulfurization processes. Among these measures, a method is known in which denitration is carried out using NH.sub.3 (ammonia) as a reducing agent under the presence of a catalyst to decompose NO.sub.x into nitrogen and water. The concentration of NH.sub.3 used is usually in the range of 50 to 150 ppm, depending upon the amount of NO.sub.x in the combustion exhaust gas.
As described above, NH.sub.3 is used as a reducing agent for NO.sub.x in the denitration of a combustion exhaust gas. Therefore, in a denitrating apparatus for the combustion exhaust gas, a storage tank for liquid NH.sub.3 is usually disposed. In order to store liquid NH.sub.3, however, a low temperature and a high pressure must be maintained, and NH.sub.3 itself is poisonous. In addition, NH.sub.3 is combustible, and when mixed with air produces an explosive mixed gas. Thus, NH.sub.3 is controlled under various laws and regulations in Japan, such as laws related to fire regulations, handling of high pressure gases, and control of noxious odor. Therefore, in the denitration step in which NO.sub.x is removed from the combustion gas, special storage and feed facilities for NH.sub.3, which is difficult to handle, are required.
On the other hand, in the above-mentioned CO.sub.2 removal step, ammonia (NH.sub.3) is detected, even though its amount is small, in the combustion exhaust gas which has been subjected to a CO.sub.2 removal treatment. It may be presumed that the NH.sub.3 is generated by the decomposition of part of alkanolamine in the process system. If NH.sub.3, though present in small amounts, is discharged into the atmosphere together with the CO.sub.2 -free exhaust gas without any additional treatment, another environmental problem may occur. Therefore, NH.sub.3 has to be recovered and disposed without any environmental harm.
As mentioned above, the concentration of an aqueous solution of monoethanolamine (MEA), which is a kind of alkanolamine, for absorbing CO.sub.2 is normally up to about 30% by weight, and it has not been used to remove CO.sub.2 from a combustion exhaust gas under atmospheric pressure at any concentrations higher than 35% by weight. The reason for this may be the fact that known techniques for removing CO.sub.2 from the combustion exhaust gas by the aqueous MEA solution under atmospheric pressure are limited, and that when the aqueous MEA solution at a high concentration is used, it is feared that a perceptible amount of the valuable MEA is discharged and lost from a CO.sub.2 removing column together with the combustion exhaust gas from which CO.sub.2 has been removed. Furthermore, even if the aqueous MEA solution is used at a high concentration, it has been considered that energy saving is scarcely expected at such concentrations as 20 to 30% by weight.
In a process for removing CO.sub.2 from a combustion exhaust gas by an aqueous MEA solution, it is desirable to decrease energy used in the process as much as possible. In particular, the aqueous MEA solution is circulated and repeatedly used in the system, and therefore it is beneficial to decrease the volume of the aqueous MEA solution for the purpose of reducing an electric power consumption of a pump or the like. Moreover, in order to regenerate a reusable aqueous MEA solution from the aqueous MEA solution having absorbed CO.sub.2, a large amount of heat energy is necessary, and therefore it is important from an economical viewpoint to reduce the consumption of heat energy.